1. Field of the Invention
The present invention relates to a motor controller for sensorless driving of a brushless motor. A brushless motor is, for example, used as a source of steering assist force in an electric power steering apparatus.
2. Description of Related Art
A motor controller for drive controlling a brushless DC motor is generally configured to control the supply of a motor current in accordance with the output of a position sensor for detecting the rotational position of a rotor. However, the environmental resistance of the position sensor becomes an issue. Moreover, an expensive position sensor and wirings related to such position sensor obstruct reduction of cost and miniaturization. Thereupon, a sensorless driving system that drives a brushless DC motor without using a position sensor has been proposed. A sensorless driving system is a method for estimating the phase of a magnetic pole (an electrical angle of a rotor) by estimating the induced voltage caused by the rotation of a rotor.
Since induced voltage cannot be estimated when the rotor is not in rotation and when the rotor is rotating at an extremely low speed, the phase of the magnetic pole is estimated by other methods in those situations. Specifically, as shown in FIG. 2(a), a high frequency exploration voltage is applied on the U-phase stator coil 51, the V-phase stator coil 52 and the W-phase stator coil 53. A high frequency voltage vector (its magnitude is constant) that rotates along the direction of rotation of the rotor 50 is formed about the origin of the α-β coordinate, which is a fixed coordinate that assumes the rotation center of the rotor 50 as the origin. The high frequency voltage vector is a voltage vector that rotates in sufficiently high speed relative to the rotation speed of the rotor 50. With the application of this high frequency voltage vector, an electric current flows to the U-phase stator coil 51, the V-phase stator coil 52 and the W-phase stator coil 53. The electric current vector that expresses the magnitude and the direction of the electric current of these three phases on the α-β coordinates rotates about the origin.
The inductance of the rotor 50 have different values in d-axis and in q-axis. The d-axis is a magnetic pole axis along the direction of magnetic flux, and the q-axis is perpendicular to the d-axis (an axis that along with the direction of torque). Therefore, the magnitude of the electric current vector is large in the case of the direction close to the d-axis, and is small in the case of the direction close to the q-axis. As a consequence, the endpoint of the electric current vector draws an oval trajectory 55 on the α-β coordinates, taking the direction of d-axis of the rotor 50 as the major axis, as shown in FIG. 2(b).
Therefore, the magnitude of the electric current vector takes local maximum values in the directions of the N-pole and the S-pole of the rotor 50. That is, the magnitude of the electric current vector has two local maximum values in a single cycle of the electric current vector, as shown in FIG. 3(a). In this case, when the magnitude of the electric current vector is sufficiently large, the magnitude of the electric current vector in the direction of the N-pole takes the maximum value (cf. curve L1). This is because the inductance is smaller at the N-pole side than at the S-pole side of the rotor 50 due to the influence of a magnetic saturation of the stator.
Thus initially, a sufficient magnitude of high frequency voltage vector is applied, and the local maximum electric current vector corresponding to the N-pole is specified. Subsequently, a high frequency voltage vector that is smaller in magnitude is applied, thereby to estimate the phase of the rotor 50 based on the local maximum value of the electric current vector. More specifically, the phase angle (electrical angle) θ of the rotor 50 can be obtained from an α-axis component Iα and a β-axis component Iβ of the electric current vector when the magnitude takes the local maximum value, by θ=Tan−1 (Iα/Iβ).
However, as shown in FIG. 2(b), distortion occurs to the response of the electric current at the application of high frequency voltage vector due to the difference between the inductances in the direction of d-axis and q-axis. As a consequence, a computer needs to conduct a complicated computing process in order to obtain the α-axis component Iα and the β-axis component Iβ of the electric current vector. As a consequence, there is a problem in which computing load on the processor is heavy.